This disclosure relates generally to electronic display technology and more specifically to multi-view three-dimensional parallax displays.
It is known that it is possible to create a three-dimensional image by approximating the wavefronts that emanate from three-dimensional (3-D) scenes. One such class of displays contains “parallax displays,” which project the appearance of a 3-D scene from multiple viewpoints. Parallax displays generally allow a viewer to move his head horizontally and/or vertically to inspect the 3-D scene from different viewpoints. FIG. 1 illustrates a generalized parallax display 10.
In FIG. 1, a 3-D image 20 is projected by the parallax display 10 due to rays emerging from image plane (or hologram plane) 40 which enter the eyes of a viewer at location A or B. In general, an illumination source 50, which is typically a collimated laser beam, a sequence of 2D bitmapped images, or a single 2D image composed of interdigitated 2D views, passes light through a light steering and shaping element 55.
There are several specific ways to construct parallax displays. One approach utilizes a lens sheet, such as a lenticular lens array or a holographic optical element of similar function, to map a field of interdigitated images to their corresponding nearest viewpoints. In this way, a user walking around the parallax display will see a series of images that approximate the scene's appearance from the corresponding viewpoints. In FIG. 2, a lenticular lens sheet 52 includes an array of lenticular lenses 54 on at least one of its surfaces. Lenticular lens sheet 52 enables the parallax display 10 to project different imagery for different viewing angles. If properly registered imagery is projected onto the screen, or if the screen is overlaid on an image source such as a liquid crystal display (LCD), the system will provide imagery that provides correct perspective and parallax and also has variable transparency so that objects may occlude each other. This requires computing image data from several viewpoints for each projected frame. Though lenticular lenses and lens arrays are well known in the art, a brief description of how they work will be provided.
A widely known embodiment of a lenticular lens array is a lenticular lens sheet. It includes a sheet with a plurality of adjacent, parallel, elongated, and partially cylindrical lenses and multiple (e.g. two) interleaved lenses on the sheet. In general, the plurality of lenses enables the multiple interleaved images to be displayed on the underlying sheet but only one of the images will be visible from any given vantage point above the sheet.
The underlying principle which explains this is illustrated in FIG. 2, which presents a schematic side view of a lenticular lens sheet 52 with a plurality of lens elements 54(1-3). The image on the underlying sheet is represented by pixels 56-58. In this example, three image pixels, identified by suffixes “a”, “b”, and “c”, respectively, are shown under each lens element 54. Thus, for example, under lens element 54(1) there are three associated pixels, namely 56a, 56b, and 56c. 
If a person views the sheet from location “A”, lens element 54(1), because of its focusing ability, allows that person to see light from pixel 56a. That is, of the light which lens element 54(1) collects, it only sends toward the person at location “A” that light which is collected from pixel element 56a. The rest of the light which lens element 54(1) collects from other locations under the lens is sent off in other directions and will not be seen by a person at location “A”. For similar reasons, a person at location “B” only sees light emanating from pixel 56b, but does not see light emanating from other locations under lens element 54(1).
In U.S. Pat. No. 5,172,251, Benton and Kollin disclose a three dimensional display system. More recently, Eichenlaub et al (Proc. SPIE, 3639, p. 110-121, 1999) disclosed a discrete light field display, which produces up to 24 discrete viewing zones, each with a different or pre-stored image. As each of the observer's eyes transitions from one zone to another, the image appears to jump to the next zone.
In practice, parallax displays are problematic. In general, there is significant noticeable light emitted in inappropriate directions, causing imagery from wrong viewpoints to be visible. Furthermore, image realism is reduced because practical constraints limit the number of views that can be handled by each lens element. For example, the pixel density and the number of viewpoints are bounded by diffraction effects and brightness requirements. Also, many known lenticular sheet parallax displays produce undesirable dark bands as the viewer transitions between viewpoints. Therefore a parallax display with a large (i.e., 100+) number of viewpoints, high resolution, high brightness, and smooth transition between view zones is desired.
It is necessary to more closely approximate the light field generated by a 3D scene than by using lenticular sheets. A subset of the parallax display set contains holographic displays and holographic stereograms. A holographic video (“holovideo”) system creates 3D imagery that looks realistic, appears to float inside or outside of a viewing zone or panel, and exhibits motion parallax. Holovideo provides the monocular and binocular depth cues of shading, motion parallax, and viewer-position-dependent reflection and hidden-surface removal.
One group of systems was created at the Massachusetts Institute of Technology (MIT) Media Laboratory that in general creates holographic video by scanning the image of an acousto-optic scanner over a vertical diffuser. This is illustrated in FIG. 3.
An idealized holographic stereogram emits light from each holographic pixel (or “hogel”) in a way that allows a horizontally-moving viewer to see a continuous range of perspectives. See FIG. 5A. Here, the hologram plane 340 is decomposed into hogels such as hogel 341. A continuous range of viewer locations is shown.
Existing synthetic holographic stereograms sample the parallax views. Sampled parallax is shown in FIG. 5B. Scene parallax is captured from a finite set of directions, and is then re-projected back in those same capture directions. In order to prevent gaps between parallax views in the view zone, each view is uniformly horizontally diffused over a small angular extent.
Two things are needed to generate a holographic stereogram in this fashion: a set of N images that describe scene parallax, and a diffraction pattern that relays them in N different directions. In the case of the MIT Media Laboratory's holographic video system, a set of N diffractive elements, called basis fringes, are computed. When illuminated, these fringes redirect light into the view zone as shown in FIG. 6. These diffractive elements are independent of any image information, but when one is combined with an image pixel value, it directs that pixel information to a designated span in the view zone. FIG. 6 shows three basis fringes, 355, 360, and 365 for three spatial frequencies. To the right of each basis fringe is shown an example of repeating that basis fringe across a hologram line. Basis fringe 355 is repeated across a hologram line 342 and is illuminated by illumination 350, resulting in output 356 with a trajectory determined by basis fringe 355. Likewise, basis fringe 360 of higher frequency is repeated across a hologram line 343 and is illuminated by illumination 350, resulting in output 361 with a different trajectory and similarly for 365.
There are several ways to infer what basis fringes are required to generate a 3D scene. A typical method is to capture a scene using computer-graphic methods from N different directions. This method is illustrated in FIG. 7. In FIG. 7, to capture or render scene parallax information, cameras are positioned along a linear track, with the view also normal to the capture plane. N views are generated from locations along the track that correspond with center output directions of the basis fringes. In this type of horizontal parallax only (HPO) computed stereogram, correct capture cameras employ a hybrid projection—perspective in the vertical direction and orthographic in the horizontal. A desired 3D scene 2 is positioned near a capture plane 4. A set of cameras, C0, C1, and CN-1, are illustrated taking snapshots of the scene 2 from a series of viewpoints.
Once N parallax views have been generated, the MIT group combines them with the N pre-computed basis fringes to assemble a holographic stereogram. In practice, this scene reconstruction is achieved using the apparatus illustrated in FIG. 3. The acousto-optical modulators (AOM) produce a stream of weighted linear combinations of basis vectors, as a function of the data compiled from the step illustrated in FIG. 7.
As described, the handful of existing systems decompose a synthetic hologram plane into spectrally-homogenous regions called hogels, each of which is “projected” in its entirety by a spatial light modulator (SLM) or acousto-optical modulator. An acousto-optical modulator is a device which, in one mode of operation, can diffract light when an ultrasonic sound wave propagates through it. Because holograms may require 1000 line pairs per millimeter, the imagery is usually small, or of low resolution.
It is well known that computational techniques enable the creation of synthetic holograms. Typical holograms require roughly 300 to 2000 lines per mm (ten million samples per square millimeter) for practical diffraction of visible light. This has been a difficult obstacle.
It is computationally difficult to generate the AOM inputs that result in the desired light field. Furthermore, the system uses components such as acousto-optic scanners and galvanometric scanners which are financially prohibitive. This type of system is shown in FIG. 3. A laser 150 illuminates a back of AOMs 154. The AOMs operate in a mode that diffracts the laser light horizontally, generating the constituent “hogels” of the final holographic image. Vertical and horizontal scanners throw the diffracted light to a vertical diffuser 159. An image volume 30 straddles the vertical diffuser 159. The 3D light field is visible by a viewer.
Another method of holographic video uses groups of SLM pixels as holographic fringes. One embodiment of this is described in C. Slinger, B. Bannister, C. Cameron, S. Coomber, I. Cresswell, P. Hallett, J. Hughes, V. Hui, C. Jones, R. Miller, V. Minter, D. Pain, D. Scattergood, D. Sheerin, M. Smith, and M. Stanley, “Progress and prospects for practical electro-holography systems,” in Practical Holography XV and Holographic Materials VII, Stephen A. Benton, Sylvia H. Stevenson, and T. John Trout, eds., Proceedings of SPIE v. 4296 (2001). This is depicted in FIG. 4. A laser 250 illuminates an electrically-addressable spatial light modulator (EASLM) 251. An optically addressable spatial light modulator (OASLM) 253 is addressed by a time series of images from EASLM 251 by a replication/relay stage 252. In this way, the speed of the EASLM is traded-off for spatial resolution on the surface of the OASLM. The imagery projected onto the OASLM is a hologram. Each pixel of the EASLM and OASLM are used as constituents of the underlying diffraction patterns. Electrically-addressable SLMs and replication optics project computer-generated hologram image segments onto an optically-addressable SLM (OASLM) for readout. The system requires a high-powered laser and generates low-resolution imagery because each SLM pixel is used within holographic fringes.
In summary, many existing holographic video systems can be schematically illustrated as shown in FIG. 10. A set of basis fringes 355, 360, and 365 are weighted using known techniques after scene generation by a “hogel vector” to form a stream of hogels 370. A “hogel vector” is a sampled hogel spectrum, specifying the diffractive purpose of a hogel. A scanning or tiling system, 154, scans this over hologram plane 340 to generate an image volume 30.
The systems described above suffer from low resolution, demanding computational requirements, and the utilization of costly optical and mechanical components. A system made of volume-reproducable components and existing computational infrastructure is desired which is capable of high-resolution imagery and the ability to better approximate 3D light fields.